1. Field of the Invention
The invention relates to a magnet assembly and particularly to an active-shield superconducting magnet assembly.
2. Description of the Prior Art
An active-shield assembly is one in which an inner coil system and an outer coil system are connected in series to create magnetic fields in opposing directions so that a strong uniform magnetic field is created at the center of the coil systems, but the stray field prevailing outside the coil systems is very small.
Such an assembly is described in European Patent Publication No. 0144171, the contents of which are incorporated herein by reference. In the assembly described in EP-A-0144171, the coil systems creating the opposing magnetic fields are wound from superconducting wire. Superconducting coil systems are used in order to achieve strength and high precision of the magnetic field created in the center of the coil systems. A prime application of these magnet assemblies is in NMR (nuclear magnetic resonance) imaging where these qualities are of vital importance. The superconducting condition of the coils is achieved by cooling the coils to very low temperatures, of the order of 4.2 K, using cryogenic techniques. In the superconducting condition, the wires can carry a very high current with negligible energy losses in the wire.
A coil in a superconducting state can convert to a normal conducting state, the transition being commonly referred to as a quench. A quench may occur unintentionally due to local disturbances or structural deficiencies or it can be induced intentionally (for example, by means of local electrical heaters) as a way of rapidly reducing the magnetic field. This might be needed for example in a case where it is necessary to give urgent treatment to a patient undergoing an NMR procedure in the magnet assembly. When a quench occurs, there is a rapid increase in resistance in the quenched part of the coil which causes the energy stored in the coil system to be converted into heat. The heat is conducted to adjacent parts of the coil, causing these parts to quench. As an increasingly large part of the coil is quenched, the temperature associated with the heat energy increases, causing the resistance of the coil to rise further and accelerating the quench process until all the stored magnetic energy is converted into heat. This can occur extremely quickly, typically in about 10 to 20 seconds.
The increased resistance in the coils causes the current carried by the coils to decay. As the current in the coils decays, the magnetic field provided by the coils alters rapidly. In known magnet assemblies, the magnets are wound onto formers which are generally of aluminum, and the coils are housed within radiation shielding casings which are suspended in a cryostat housing. The casings and housing are formed of a material having a high thermal conductivity such as aluminum and tend also as a result to have a high electrical conductivity. The altering magnetic field couples with the electrically conductive components in the magnet assembly and cryostat structure. In particular, it couples strongly with the formers on which the coils are wound, and the radiation shielding casings within the cryostat. This magnetic coupling induces currents in these components, which currents are sufficiently large to generate significant magnetic fields.
As described in EP-A-0144171 the coils of an active-shield magnet assembly are designed not only to produce a uniform magnetic field at the center of the assembly but also to produce an external magnetic field which is as low as possible as close as possible to the assembly. The external magnetic field is termed herein the "stray field" and is commonly specified in terms of an ellipsoidal or cylindrical volume outside which the magnetic field due to the magnet assembly nowhere exceeds a specified level. This provides a way of denoting a safety zone a certain distance from the magnet.
The other components of the magnet assembly are, however, designed according to normal thermal and structural considerations, so that the distribution of currents induced in them during a quench is uncontrolled. In normal circumstances this is not a problem, since no currents flow in these components during the steady state operating condition. During a quench, however, significant, uncontrolled currents can be induced as explained above. The magnetic field created by the induced currents upsets the balance between the central magnetic coils and the shielding magnetic coils. The result can be a temporary but significant increase in the stray field during a quench, which could present a risk to people or equipment located in the safety zone close to the positions for which a steady maximum stray field value is specified under normal circumstances. This is clearly undesirable, particularly in hospitals where sensitive equipment might be located in the nominal safety zone.